Mechanical pulps can be used in furnishes for the manufacture of business forms, writing papers, and high grade publication papers for books, which are all long-life uses requiring paper that does not yellow with age. Mechanical pulps include groundwood (GW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), chemimechanical pulp (CMP), variations thereof (e.g., stone GW, pressurized GW, thermo-RMP, pressure RMP, pressure TMP, chemi-RMP, long fiber CMP, thermomechanical chemi pulp); recycled pulp; and compositions containing mechanical, chemical and recycled pulps.
Papers, however, made with mechanical pulps are known to turn yellow during use. This yellowing restricts their use to applications requiring only a short-life for the paper. If the time taken before yellowing of these papers begins could be increased, the potential market for bleached TMP and CTMP would be expanded significantly, for example, more bleached TMP and CTMP could be included in mixed (e.g., kraft-mechanical or sulfite-mechanical) furnishes used to manufacture high brightness papers. Displacing significant amounts of more expensive fully bleached, low yield chemical pulps with less expensive high yield mechanical pulps promises significant economical benefits.
Photoyellowing occurs primarily in finished paper. It is thought that photoyellowing results mainly from radical photochemical reactions of residual lignin in pulp. Therefore, high-lignin pulps and products containing such pulps are more susceptible to brightness loss than more expensive, low-lignin pulps. Phenoxyl, hydroxyl, alkoxyl and peroxyl radicals are likely intermediates in the process. Consequently, radical scavengers and hydrogen donors/antioxidants provide protection against photoyellowing. Photoexcitation of α-carbonyl groups often triggers a chain of radical reactions, and chemical modification of such groups as well as absorption of light energy by optical (UV) screens/absorbers affect discoloration significantly. The known classes of chemicals that provide limited protection against photoyellowing of mechanical pulps include thiols, stable nitroxide radicals, sterically hindered hydroxylamines, phosphites, dienes, aliphatic aldehydes, and UV screens. Usually, the amounts of chemicals required for adequate protection are not economically feasible and these compounds usually carry other undesirable traits, such as high toxicity and unpleasant odors. A need therefore exists for a method of decreasing the rate of photoyellowing that is non-toxic and economical.